Our Radiation Oncologist recently showed me a pelvis CT image of a male patient who had undergone bilateral hip replacement with forged titanium alloy implants. The patient had early stage prostate cancer, and the doctor wanted to treat this patient with IMRT. The difficulty was to delineate the prostate, seminal vesicles and other organs at risk because of the streak artifacts in the CT images due to high-Z material in the patient. The question came up about how this kind of artifact can be removed or minimized so that the target organ, as well as the organs at risk and other organs can be delineated for treatment planning. I did some research and was able to come up with some good articles on this topic. I am summarizing my findings below for those who are interested to learn or as a reference for those who may have such cases in the clinic.

Streak artifacts in CT images are generated in conventional CT when implanted objects of high atomic number exist in the patient. The artifact and image degradation associated with the kilovoltage (kV) CT imaging in the presence of high atomic number material greatly hinders the ability to delineate tumors and certain organs, particularly in the treatment planning of a prostate patient with hip prostheses. Such a situation, therefore, precludes precise dose calculation. There are several techniques reported that, if used, can minimize such artifacts, thereby enhancing image visualization for the delineation of tumor and other organs.

1- Charmley et al. (1) suggested that the use of CT-MR image registration to define target volumes in pelvic radiotherapy in the presence of bilateral hip replacements could facilitate target definition of prostate patient with hip replacements. However, a number of factors were found to affect image quality and/or the accuracy of target definition. The standard MR couch, different from a CT or linac treatment couch, might result in different patient positions, and the presence of the metallic implants may create significant distortion.

2- Yazdia M. (2) suggested an adaptive approach to metal artifact reduction in helical computed tomography for radiation therapy planning. At that time, they may require manual image post-processing and most CT scanners available in radiation oncology department are not equipped with these features.

3- The artifact image and degradation associated with the kilovoltage (kV) CT imaging in the presence of high atomic number material is greatly reduced with Megavoltage Cone Beam Computed tomography (MV-CBCT). MV-CBCT has been used in image-guided radiotherapy (IGRT) to correct patient setup immediately before treatment. Hansen et al (3) used this technique to treat paraspinous tumors in the presence of orthopedic hardware. It allows rapid acquisition of 3D images that can be registered with the planning CT with millimeter precision and enhance image visualization by exploiting the predominantly Compton scattering of high-energy photons delivered in the MV-CBCT system. Aubin et al. (4) of the Department of Radiation Oncology at UCSF did a study with the support of Siemens Oncology Care systems on the use of Megavoltage Cone Beam CT to complement CT for target definition in pelvic radiotherapy in the presence of hip replacement. They found the MV-CBCT image could be used to clearly visualize the hip prostheses and provide sufficient soft-tissue contrast to help delineate the prostate, bladder and rectum. The artifacts on the kV CT obscure the border between the prostate and anterior wall of the rectum and the interface between the prostate base and the bladder neck. However, the MV-CBCT images were particularly useful to help delineate these structures as well as the lateral extension of the prostate in the axial plane, the seminal vesicles and the lymph nodes. Also, normal anatomy such as pelvic bones, penile bulb, bladder, femoral heads, rectum and small bowel can be delineated with higher accuracy as well. They evaluate this technique for seven patients. For each patient, the MV-CBCT images were imported into the treatment planning system and registered with the original CT using body anatomy contoured on each image set. The target volumes and organs at risk for prostate treatment were contoured using both the CT and the MV-CBCT for single hip replacement, and using only the MV-CBCT for bi-lateral hip prostheses. For the full article, click on: http://bjr.birjournals.org/cgi/reprint/79/947/918

The following two figures taken from Aubin M. at el (4) show the difference between conventional CT and MV-CBCT images:

Aubin M. at el. Use of Megavoltage Cone-Beam CT to complement CT for target definition in pelvic radiotherapy in the presence of hip replacement. Short Communication: British Journal of Radiology 2006.